Add-ons allow you to add extra features and functionality to Firefox, modify the Firefox user interface and change its appearance. There are several types of add-ons but extensions are the most common. Anyone can create an extension and make it available for download.

In the past, extensions often stopped working each time a new version of Firefox was released, because developers had to update them every six weeks to keep them compatible. Since extensions could also modify Firefox internal code directly, it was possible for bad actors to include malicious code in an innocent-looking extension.

To address these issues, and as part of broader efforts to modernize Firefox as a whole, we’ve transitioned to a new framework for developing Firefox extensions. Extensions created with the new standard are safer, more secure, and won’t break in new Firefox releases. You can still personalize Firefox with extensions the same way you do now, except they won’t break in new Firefox releases.

Note: Starting with Firefox version 57 released on November 14, 2017, only extensions built with this new technology will work in Firefox. A majority of the most popular add-ons are already compatible, so most users should not notice any changes.

If any incompatible extensions are installed, they will be shown in the Add-ons Manager as Legacy Extensions. Click the menu button and choose Add-ons (or enter about:addons in a new tab) to open the Add-ons Manager. Then click on Legacy Extensions. From there, you should be able to find recommended replacements for any legacy extensions. You can read more about this feature in this Add-ons blog post.